The present invention relates to implantable devices, and more particularly to implantable medical devices having a permanent magnet therein that aligns an external (non-implanted) coupling device with the implantable device so that an electromagnetic signal may be optimally coupled between the two devices.
It is known in the art for an implantable medical device, e.g., implantable cochlear stimulator (ICS), to have a permanent magnet placed therein. An external (non-implanted) device, e.g., a headpiece of an ICS system, also has a permanent magnet placed therein. Both the external device and the implantable device may have coils mounted therein to allow power and information to be electromagnetically coupled, e.g., inductively coupled, between the two devices. Optimum signal and power coupling occurs when the implanted coil in the implantable device and the external coil in the external device are properly aligned. The magnetic attraction between the two permanent magnets, one located in the implantable device (typically in the center of the implant coil) and the other located in the external device (also typically located in the center of the external coil), magnetically hold the external device in a proper position so that the needed alignment between the two devices is maintained. See, e.g., U.S. Pat. Nos. 4,352,960 (Dormer et al.) and 4,726,378 (Kaplan), both of which patents are incorporated herein by reference.
It is also known in the art to replace one of the magnets, e.g., the magnet in the implant device, with a "keeper". A keeper is generally made from a material which is not magnetic, per se, i.e., is not magnetized, but which provides a low reluctance magnetic path for magnetic flux. The use of a keeper advantageously improves the resistance of the implant system to MRI magnetic fields, in the event such MRI fields should be applied to the patient. That is, should MRI (magnetic resonance imaging) be conducted on the patient having the implant, the magnet in the implant becomes demagnetized and distorts the MRI image. A keeper, if used in place of the implant magnet, is not damaged by the MRI and distorts the MRI image less.
Disadvantageously, when power and signals must be electromagnetically transferred between the implant device and the external device, placing a magnet or keeper within the coils used for the power and signal transmission absorbs power and reduces the efficiency of the system. Here, system efficiency is defined as the ratio of input power delivered to a transmission coil input to output power recoverable at a receiving coil. What is needed, therefore, is a system that allows magnetic attraction to maintain a proper alignment between an implantable device and an external device without having the magnetic elements absorb large amounts of power, thereby making the system more efficient. What is further needed is such a system that is compatible with MRI, i.e., that allows MRI to be conducted when necessary without harming the implant device and without significantly distorting the MRI image.